Anaerobic digestion


Introduction :

Today the problem of waste pollutants has increased due to increasing number of industries as well as urbanization. Water is polluted in large amount due to these waste pollutants. These waste products can be from different sources food waste, industrial waste, agricultural waste, urban waste etc. Waste pollutants which are obtained from these sources are usually organic materials.

The process of anaerobic digestion aims on breakdown of these waste materials. "It is a complex biochemical process of biologically mediated reactions, by a consortium of microorganisms to convert organic acids to methane and carbon dioxide."

The process stabilizes the waste by reduction in its mass, reduction in odour and reduction in number of pathogens present in it. The process helps in reduction of landfill gas in the atmosphere. It produces energy in the form of biogas. Biogas contains methane and carbon dioxide. Digestate or waste produced after the process is over is rich in nutrients and can be used as fertilizers.

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General equation of the whole process is

The process :

The process was divided into three stages namely Hydrolysis, acid formation and gas formation (Lawrence and McCarty 1967). Stage of acid formation can be termed as non-methanogenic phase and gas- formation as methanogenic phase. A large number of bacterial population is involved in these processes.

Different species of microorganisms which are used in anaerobic digestion vary according to the materials degraded by them. Microorganisms involved use different nutrients as electron acceptors. For example nitrate is used by Paracoccus denitrificans, Pseudomonas stutzerii; Costridium aceticum and Acetobacteium woodi use carbonate. Cellulolytic microbes, responsible for hydrolyzing cellulose, are present in abundance in anaerobic digesters.

It is estimated that in 1st and 2nd phase of degradation 128 ordersof 58 species and 18 genera are involved eg. Ruminococcus, Eubacterium. 3rd and 4th phase involve 81 different species from 23 genera. Eg. Methane bacteria. Some carbohydrates are also degraded by homoacetogenic bacteria which uses acetyl CoA pathway.

There are different specific genera responsible for particular phases.

Hydrolytic phase :

It is the phase in which undissolved organic compounds/ polymers are converted to simple monomers which are water soluble, by the action of facultative or obligate anaerobes. Compounds like starch, cellulose, lignin, pectin are broken down into simpler compounds by the action of microbial enzymes. Different exoenzymes/hydrolases are responsible for this breakdown. Generally covalent bonds between these polymers are broken. Facultative anaerobes take oxygen from h2o and produce a low redox potential.

There are many genera which participate in this phase.

  • Genus Bacterioides : B. uniformis, B. acidifaciens, B. ruminicola. Immobile gram-negative rods, which convert carbohydrates, peptones, organic acids, amino acids to succinate, acetate, formate etc.
  • Genus Lactobacillus : L. pentosus, L aviaries. Convert glucose to lactate or organic acids by the process of homo or heterofermentation
  • Propionibacteria which are gram positive, immobile, spore forming rods use carbohydrate s as their substrate.
  • Sphingomonas, Sporobacterium, Bifidobacterium are also involved in this step. Their role is very substrate specific.

Usually Acetate and hydrogen which is produced are used directly by methanogenic bacteria, but sometimes products with high chain lengths are formed. They are converted into simple compounds in next step.

Acidogenesis :

It can be said that acidogenesis process works parallel to the hydrolytic stage. It is the process in which complex molecules like starch, cellulose are depolymerized by the action of enzymes like lipases, proteases, amylases. After that these products undergo fermentation to form volatile fatty acids (propionate, butyrate), compounds like methanol along with gases like ammonia, hydrogen and carbon dioxide.

Nearly all the bacteria which are present in hydrolytic stage are present in this step.

  • Clostridium- different species of clostridium act on different substrates. eg. C. butyricum - starch, pectin; C. acetolerans - Xylan and glucose
  • Ruminococcus - Carbohydrate fermentation to form acetate, formate, succinate, lactate, CO2, H2 . eg. R.gnavus - conversion of sugar to ethanol; R. albus - acts on cellulose and different sugars.
  • Paenibacillus - They produce lactate, formate, propionate as the product of the reaction.

Acetogenesis :

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It is a very important stage since all the metabolites which are formed so far are converted to acetate, hydrogen and carbon dioxide.

Compounds like volatile fatty acids are anaerobically oxidized by acetogenic bacteria by the process of ß- oxidation. Final products of this stage are hydrogen and acetate. Homoacetogenic bacteria act on hydrogen and carbon dioxide formed to produce acetate. Hydrogen inhibits the metabolism of acetogenic bacteria.

There is an association between acetogenic bacteria which produce H2 and Methanogenic bacteria which consume H2, Which helps in maintaining the thermodynamic balance. It can be concluded that acetate formation depends on the syntrophic association between acetogenic and methanogenic bacteria.

Methanogenesis :

Methanogenesis is the final step of anaerobic digestion. Methane and CO2 are formed as the endproduct of this step. There are two ways in which methanogenic bacteria produce CH4. They either use carbon dioxide or acetic acid as terminal electron acceptor.

Methanogenic bacteria are very substrate specific. Some use formate or acetate as C source, Some autotrophic bacteria use CO or CO2 as C source. Some methanogens can obtain carbon from sources like formate, dimethyl sulfide etc. Some methanogens work in presence of certain cofactors for eg. Methanogenic archea nedd cofactor F420 , and they act in presence of hydrogenase.

References :

  • Anaerobic digestion of biomass by David p. Chyenoweth, Ron Isaacson.